A thermally conductive sheet is generally disposed between a radiating body such as a heat sink and a heat-generating part for electronics or electronic parts including an integrated circuit (IC). Such a heat conductive sheet has conventionally contained a thermally conductive filler such as alumina or aluminum hydroxide.
In order to allow the thermally conductive sheet to exhibit superior thermal conductivity, higher amount of a thermally conductive filler is desired to be filled uniformly in the thermally conductive sheet. The thermally conductive filler is subjected to a surface treatment generally with a surface-treating agent such as silane coupling agent, titanate type coupling agent, or aluminate type coupling agent to fill the filler in the thermally conductive sheet uniformly with high concentration (see JP-A-11-209618 and JP-A-2003-213133).
However, kneading and heat treatment for a long period of time is generally necessary to sufficiently subject the thermally conductive filler to a surface treatment with a surface-treating agent including these coupling agents. Thus, a surface treatment of a thermally conductive filler with a surface-treating agent is complex and sometimes needs long time. This causes an increase in production cost for a thermally conductive composition and a thermally conductive sheet using a thermally conductive composition (see JP-A-11-209618, JP-A-2003-213133, JP-A-5-171057, and JP-A-2001-302234).
A thermally conductive composition obtained with insufficient kneading of a surface-treating agent and a thermally conductive filler or with insufficient heat treatment cannot easily be molded into a predetermined shape or applied on a support such as a liner. In addition, a thermally conductive sheet obtained by using such a thermally conductive composition sometimes has a problem of heat aging or bleeding of a surface-treating agent (coupling agent).
Meanwhile, as inexpensive surface-treating agents usable in place of the above coupling agent, there have been known higher fatty acids, higher fatty acid esters, and low molecular weight surfactants, and the like (see “Kneading Technique for Resin/Filler System”, by Technical Information Institute Co., Ltd., Mar. 31, 2001; and JP-A-2004-2527). However, thermally conductive compositions using these surface-treating agents do not have sufficiently improved moldability and has a problem of difficulty in increasing an amount of a thermally conductive filler in the thermally conductive sheet to be obtained.
There has also been proposed a method using a polymer containing a polar group or a high molecular weight dispersant as a surface-treating agent (see “Kneading Technique for Resin/Filler System”, by Technical Information Institute Co., Ltd., Mar. 31, 2001; JP-A-5-320414; JP-A-2002-284884; JP-A-2004-59851; and JP-A-2004-10859). However, when the polar group of the polymer containing a polar group is cationic, the polymer is required for neutralization with strong acid when it is used as disclosed in JP-A-5-320414. Therefore, there arises a problem that sufficient care should be taken against corrosion of a device or the like. When a solid or rubber-like polymer is used as the polymer as disclosed in “Kneading Technique for Resin/Filler System” or JP-A-2002-284884, conditions for use are remarkably restricted disadvantageously in comparison with the case of using a liquid surface-treating agent.
Further, JP-A-2004-59851 and JP-A-2004-10859 disclose thermally conductive sheets each using copolymer having a predetermined acid number or a high molecular weight dispersant. However, an olefin-maleic anhydride copolymer is solid at around ordinary temperature and has a problem of low compatibility with various resins and monomers.
Meanwhile, with regard to a thermally conductive sheet using a non-silicone resin as a binder component, an organic phosphorous compound such as phosphates is generally blended, as a flame retardant, in a thermally conductive composition constituting the thermally conductive sheet to impart flame retardancy to the sheet. However, such an organic phosphorous compound is easily hydrolyzed, which sometimes causes deterioration of the thermally conductive sheet, and sometimes an offensive odor is easily emitted.